In recent years, a magnetic disk drive has become increasingly smaller in size and larger in capacity and is required to achieve higher recording density. To increase the recording density, the unit recording area should be reduced while the detection sensitivity of a weak magnetic signal should be improved. For this purpose, technological development for further lowering the flying height of a magnetic head has advanced. In such a case, if the surface roughness of a substrate is large, the magnetic head can collide with a disk surface during reading and writing operations. Therefore, more excellent surface quality of a glass substrate is necessary to increase the recording density even further. Thus, particles containing silicon dioxide (also referred to as “silica particles” in the following) have been used as an abrasive for a final (finish) polishing liquid. Recently, as a polishing composition capable of reducing the surface roughness of the glass substrate and achieving a high polishing rate, an acidic polishing composition for a glass substrate has been proposed that includes silica particles having a small particle size and an acrylic acid/sulfonic acid copolymer having a weight average molecular weight within a predetermined range (Patent Document 1).
On the other hand, a polishing composition capable of suppressing roll-off also has been proposed that includes an abrasive such as colloidal silica and a water-soluble polymer in which at least one of a carboxylic acid group, a salt of the carboxylic acid group, a sulfonic acid group, and a salt of the sulfonic acid group is bonded to the main chain (Patent Document 2).